The invention concerns a process for fractionation of a gaseous mixture according to the pressure swing adsorption process wherein a gaseous mixture under elevated pressure is conducted in cyclic alternation through three adsorbers filled with an adsorbent, with selective adsorption of at least one first component and formation of a product gas depleted of first component, each adsorber passing through chronologically mutually shifted switching cycles, and each switching cycle comprising an adsorption phase at maximum process pressure, expansion phases conducted first cocurrently and then countercurrently to the adsorption direction, a desorption phase at minimum process pressure, and pressure buildup phases for restoring the adsorption pressure; and wherein pressure equalization takes place between an adsorber in a cocurrent expansion phase and an adsorber in a pressure buildup phase.
Such a process has been known from DOS No. 2,724,763. In this reference, a pressure swing adsorption installation has been disclosed which is operated with three adsorbers, each adsorber being associated with a purifying bed arranged upstream thereof. In the purifying bed, with the aid of an adsorbent, the components are separated from the gaseous mixture to be purified or fractionated that could lead to problems in the primary adsorber, especially because they are strongly bound by the adsorbent and can be separated again only with difficulties. Preliminary purification is particularly advantageous, for example, in the treatment of gases which contain water vapor and carbon dioxide, such as air, steam reforming gas, synthesis gas, etc., on zeolitic adsorbents inasmuch as the aforementioned impurities are very strongly adsorbed on zeolites and should be kept away from the latter as much as possible.
In the conventional method, each adsorber and the associated purifying bed pass through the same cycle. Preferably, both components are even arranged in a single adsorption vessel. Following a desorption phase, conducted in this process by evacuating to subatmospheric pressure, an expansion gas obtained during the cocurrent expansion enters the adsorber between the purifying bed and the primary adsorber, in order to effect a first pressure buildup phase. A portion of this gas enters the purifying bed countercurrently to the adsorption direction and thereby urges the otherwise advancing front of the bed still further back toward the inlet side whereas the remainder is introduced into the primary adsorber in cocurrent fashion.
The pressure buildup by an expansion gas fed cocurrently is considered especially advantageous, since it is unnecessary to provide a high purity of the expansion gas. For this reason, the adsorbers can also be loaded, during an adsorption phase, up to the breakthrough of the component to be adsorbed or of the components to be adsorbed, respectively. In case of breakthrough of the adsorption front, the provision is made that the product gas, withdrawn in the impure state in such a case, is conducted through still another, previousely repressurized, adsorber and is purified therein. In this operating phase, two adsorbers are thus connected in series.
Following the first pressure buildup phase, another pressure increase takes place to the adsorption pressure by introducing crude gas introduced into the purifying bed cocurrently to the adsorption direction. Therefore, this gas first passes through the purifying bed and then enters the primary adsorber.
One drawback of the conventional process is to be seen in that satisfactory product yields can be achieved only by conducting desorption at subatmospheric pressure. The vacuum pump required to produce the vacuum represents, however, a significant cost factor within the adsorption plant which makes itself felt not only as an initial investment but also during operation in the form of energy, servicing, and repair expenses.